That doesn't sound right at all. I mostly use mine as a TDR (50 GHz with 83651B) but I can generally reload saved calibrations from months ago, less than 5 minutes after a cold start, and still see < 50 dB return loss through 20 GHz if I look at the same load.
I'd go through the receiver channel tests in the manual and try to isolate the problem to the test set or IF box. Does S21 drift as well?
I'd go through the receiver channel tests in the manual and try to isolate the problem to the test set or IF box. Does S21 drift as well?